During an evening that was ostensibly about bonding, burying the hatchet and eliminating drama, it was still an episode of RHOBH – so, everyone left upset.

In a blog for Bravo, Vanderpump, 54, acknowledged that it wasn't the force of the slap that bothered her, but the "violation" inherent in the act. "I haven't had a hand laid on me since I was a child," she wrote.

"I want you to be on your best behavior," Vanderpump's husband, Ken Todd, told Glanville when he invited her. "I don't want anything to spoil her birthday."

"Me on my best behavior is me not talking," joked Glanville, who was on her best behavior, as she was in the midst of a 21-day cleanse – meaning no sugar or alcohol – inspired by Yolanda Foster, who asked her point-blank last week if she was an alcoholic.

But Glanville did have to work for her cake – and to earn Vanderpump's approval – by singing for the guests, including Lance Bass.

"I had a horrible season last year," the veteran reality star, 54, tells PEOPLE at the launch of EVINE Live Friday in New York City. "They threw everything at me last year, the other women. It was nasty and it was mean-spirited."

Specifically, Vanderpump had a falling out with former friend Brandi Glanville, who helped turn the rest of the cast against Vanderpump at the end of last season.

"The first time I met Eileen was in the late '80s … Eileen was huge on The Young and The Restless," said Rinna, referencing Davidson's role as Ashley Abbott, before the two worked together on Days – Davidson as the villainous Kristen DiMera and Rinna as Billie Reed.

"At this point in my life and career, I feel incredibly grateful," Davidson said about her 2014 Daytime Emmy win for best actress on Days. "I've been doing this for so long … it might never happen again for me."